1. Field of the Invention
The present invention relates to a stress and strain analysis method and stress and strain analysis equipment that enable a more detailed determination of a stress and strain state of an object.
2. Description of the Related Art
Safety from a dynamic viewpoint in various products such as airplanes, trucks, piping in power plants, bridges, and orthopedic and dental implants is a basic and indispensable element for realizing a worry-free and safe society. Also, for realization and evaluation of safety of the products from the dynamic viewpoint, a stress and strain measurement technology plays an important role.
Conventionally, a strain gauge method has been used for measurement of a stress distribution. In the strain gauge method, many strain gauges are stuck to a surface of a measurement target object, and a stress and strain distribution is determined based on an output signal from each strain gauge at the time of application of a load. With this method, quantitative measurement is possible but in the case of a wide measurement area or a narrow measurement area, a thorough measurement is impossible due to a limit on the number of gauges that can be stuck in the area.
Therefore, a photoelasticity measurement method has also been used. In the photoelasticity measurement method, a stress distribution is measured by utilizing a temporary photobirefringent property that occurs in accordance with the amount of a stress at the time of application of a load to a sample. There are various photoelasticity measurement methods such as a method in which measurement is conducted using a transparent model with a similar shape to a real sample, and a method in which measurement is conducted by forming a photoelastic coating for a surface of a real sample. Each of those methods has a feature that it is possible to measure a stress as an image.
Meanwhile, an applicant of the present invention has studied an inorganic material that emits light in response to mechanical energy and have succeeded in manufacturing a material, including a base material that is a piezoelectric body having, in particular, a wurtzite structure and an inorganic substance having a luminescence center, as described in Japan Patent Publication No. 11-120801 A (Japan Patent No. 3-265356 B). The applicant of the present invention has found that it is possible to dramatically improve the light intensity emitted from an obtained thin film by adding an inorganic substance to the base material described above and have filed a patent application. Thereafter, as a result of a further study, the applicant of the present invention has found various inorganic substances that emit light in response to such forces (mechanoluminescence material), as disclosed in U.S. Pat. No. 7,060,371, and are also conducting a study for using such substances in various fields. For instance, as disclosed in JP 2003-137622 A, the applicant of the present invention propose a technique of detecting an abnormal stress that is a harbinger of destruction of concrete by mixing a mechanoluminescence material into the concrete.
As described above [04], in the photoelasticity measurement method, a stress distribution of an object is measured as an image. For instance, in the photoelasticity measurement method by a general plane polariscope, the light intensity I observed after passing an analyzer is expressed as follows:I=I0 sin22Ψ sin2(ρ/2)  (1)where I0: incident light intensity,                Ψ: principal stress direction of a sample, and        ρ: relative phase difference.        
Here, the relative phase difference ρ and a fringe order N are expressed as follows:ρ=2παt(σ1−σ2), and  (2)N=αt(σ1−σ2)  (3)where                α: photoelasticity sensitivity, and        t: thickness of the sample.        
The photoelasticity sensitivity α is expressed by the following equation:α=2πC/λ  (4)where                C: Brewster's constant, and        λ: light wavelength.        
As expressed by Equations (1) to (4), with this technique, two information items are obtained. That is, at the time when Ψ is equal to 0 or π/2, a principal stress direction can be found from a black fringe (isoclinic line), in which a principal stress axis coincides with an optical axis at a point where I is equal to 0, and a principal stress difference can be found from a black fringe (isochromatic line) that becomes a positive integer (such as N=0, 1, 2 . . . ).
This technique, however, has a fundamental limitation in that it is impossible to obtain individual stress component values σ1 and σ2.